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When does Momentum Change?

The law of conservation
of momentum only applies if there are

no external forces.
An example of an external force
is friction.

Friction is a resultant
force that makes

objects slow down and
stop if there is no
force to balance
it.

Newton's
second law tells
us that a resultant force will cause

the motion (and therefore the momentum)
of an object to change.

What is the Equation for a Change
in Momentum?

Force = change in momentum ÷ time taken for the change.

This equation is written as

F = (mv - mu)
÷ t

where F
= Force

mv = final momentum (the one it ended up
with)

mu = initial momentum (the
one it started with)

t = time

This equation can be rearranged to give

Change in momentum
= Force x time.

or mv - mu
= F x t

The units of
momentum can be
therefore be written as

Ns called Newton seconds
(Force x time) as well as kgm/s.

An example of a change in momentum
is a moving car

being stopped by using its brakes
or by crashing into a
wall.

Links Forces and Motion Momentum Revision Questions

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